Efficiently reducing CO2 with light energy as in plants has been required from viewpoints of energy problems and environmental problems. Plants use a system called a Z-scheme in which light energy is excited in two stages. In an electrochemical reaction using this system, electrons are gained through the oxidation of water (H2O), and compounds such as cellulose and saccharides are synthesized through the reduction of carbon dioxide (CO2).
As an electrochemical reaction device developed in imitation of the Z scheme of plants, there has been known an artificial photosynthesis device which obtains a potential required for reducing CO2 from visible light. The artificial photosynthesis device is capable of producing oxygen simultaneously with the reduction process and thus is also usable as a life support system in, for example, a space station.